(Adapted from the applicant's abstract) Two strategies have been proposed to cure patients with inherited diseases of the hematopoietic system; replacement of a portion of the marrow through transplant of genetically manipulated auto-logous cells, or replacement of the bone marrow through allogeneic bone marrow transplantation (BMT). Improvements in both strategies are needed to make transplantation more effective and less toxic. The use of genetically manipulated transplanted autologous cells in the clinic is still in its infancy. Specific Aims 1 and 2 of this project will define conditions for BMT required to functionally correct mouse models of human genetic disease through transplantation of normal and transduced syngeneic marrow. Several of the human genetic diseases required transplantation at an early age to prevent serious sequelae of the genetic defects; however the toxicity of total body irradiation (TBI)-containing conditioning regimens precludes its use in very young children. Specific Aim 3 will study non-TBI (and non-busulfan) methods of achieving syngeneic engraftment system will not have a matched allogeneic donor, and mismatched allogeneic transplantation is associated with a significant risk of graft versus host disease (GVHD) and graft rejection. In specific aim 4, we plan to study novel ways to reduce GVHD and enhance engraftment of mismatched allogeneic marrow by modification of alloreactive donor T cells, and by transplantation in utero. We will also use murine models to compare the toxicity and efficacy of allogeneic versus genetically "corrected" autologous transplantation as therapy for genetic diseases. This comparison will include evaluation of the genetic defect by both clinical and laboratory parameters as well as detailed evaluation of critical aspects of post-transplant recovery. This project is strongly integrated with other projects in the SCOR grant as noted below: Project 1 (Mulligan): We will incorporate information on the importance of viral host range and titer, infection conditions, and stem cell purification pre- and post-infection into the experiments in Aim 2. Project 2 (Tenen): We will utilize rAAV vectors bearing RAG-2 and gp91, developed in Project 2, to correct mouse models of human genetic hematopoietic disease in Aim 2. We will also utilize our expertise in manipulation of murine marrow to aid in characterization of the first generation of AAV vectors in Project 2. Project 3 (Griffin): Information about different infection parameters important for correction of mouse models will be directly relevant to attempts to efficiently infect human HSC, and for the in vivo and in vitro correction of genetic diseases of HSC. Project 4 (Nadler): We will directly utilize the T-cell anergy and clonal deletion technology developed in Project 4 and apply it to the analysis and prophylaxis of GVHD in our murine BMT models, and information derived from mouse studies will form the basis of clinical trials of this technique. Project 6 (Nathan): New information about methods to achieve stable chimerism following BMT, derived from Aim 1, will be utilized in Projects 4 and 6 to facilitate engraftment and reduce toxicity of allogeneic BMT in children.